The United States and Russia say they have resolved a number of issues standing in the way of restoring a nationwide truce to Syria and opening up aid deliveries, but were unable once again to forge a comprehensive agreement on stepping up cooperation to end the brutal war that has killed hundreds of thousands.

After meeting off-and-on for nearly 10 hours in Geneva on Friday, U.S. Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov could point to only incremental progress in filling in details of a broad understanding to boost joint efforts that was reached last month in Moscow.

Their failure to reach an overall deal highlighted the increasingly complex situation on the ground in Syria — including new Russian-backed Syrian government attacks on opposition forces, the intermingling of some of those opposition forces with an al-Qaida affiliate not covered by the truce and the surrender of a rebel-held suburb of Damascus — as well as deep divisions and mistrust dividing Washington and Moscow.

The complexities have also grown with the increasing internationalization of what has largely become a proxy war between regional and world powers, highlighted by a move by Turkish troops across the Syrian border against Islamic State fighters this week.

Kerry said he and Lavrov had agreed on the "vast majority" of technical discussions on steps to reinstate a cease-fire and improve humanitarian access. But critical sticking points remain unresolved and experts will remain in Geneva with an eye toward finalizing those in the coming days, he said.

"We are close," Kerry said. "But we are not going to rush to an agreement until it satisfies fully the needs of the Syrian people."

Lavrov echoed that, saying "we still need to finalize a few issues" and pointed to the need to separate fighters from the al-Nusra Front, which has ties to al-Qaida, from U.S.-backed fighters who hold parts of northwest Syria.

"We have continued our efforts to reduce the areas where we lack understanding and trust, which is an achievement," Lavrov said. "The mutual trust is growing with every meeting."

Yet, it was clear that neither side believes an overall agreement is imminent or even achievable after numerous previous disappointments shattered a brief period of relative calm earlier this year.

The inability to wrest an agreement between Russia and the U.S. — as the major sponsors of the opposing sides in the stalled Syria peace talks — all but spells another missed deadline for the U.N. Syria envoy to get the Syrian government and "moderate" opposition back to the table.

The U.N. envoy, Staffan de Mistura, briefly sat in Friday with Kerry and Lavrov. After missing an initial target date of Aug. 1, de Mistura had hoped to restart the intra-Syrian discussions toward political transition in late August. He suspended the talks in late April after a resurgence in the fighting.

Friday's meeting came a month after the Kerry and Lavrov met in Moscow and agreed on a number of unspecified actions to get the all-but-ignored truce back in force. However, as in Moscow, neither Kerry nor Lavrov would describe them in detail.

In a nod to previous failed attempts to resurrect the cessation of hostilities, Kerry stressed the importance of keeping the details secret.

"We do not want to make an announcement ... that is not enforceable, that doesn't have details worked out, that winds up in the place that the last two announcements have wound up," Kerry said. "Until we have, neither of us are prepared to make an announcement that is predicated for failure. We don't want a deal for the sake of the deal, we want a deal that is effective."

And, underscoring deep differences over developments on the ground, Kerry noted that Russia disputes the U.S. "narrative" of recent attacks on heavily populated areas being conducted by Syrian forces, Russia itself and the Iranian-backed Hezbollah militia. Russia maintains the attacks it has been involved in have targeted legitimate terrorist targets, while the U.S. says they have hit moderate opposition forces.

Expectations had been low for the talks, particularly given how efforts to forge a new U.S.-Russia understanding have fallen short virtually every month for the past five years.

At the same time, the Obama administration is not of one mind regarding the Russians. The Pentagon has publicly complained about getting drawn into greater cooperation with Russia even though it has been forced recently to expand communication with Moscow. Last week, the U.S. had to call for Russian help when Syrian warplanes struck an area not far from where U.S. troops were operating.

U.S. officials say it is imperative that Russia use its influence with Syrian President Bashar Assad to halt all attacks on moderate opposition forces, open humanitarian aid corridors, and concentrate any offensive action on the Islamic State group and other extremists not covered by what has become a largely ignored truce.

For their part, U.S. officials say they are willing to press rebels groups they support harder on separating themselves from the Islamic State and al-Nusra, which despite a recent name change is still viewed as al-Qaida's affiliate in Syria.

Those goals are not new, but recent developments have made achieving them even more urgent and important, according to U.S. officials. Recent developments include military operations around the city of Aleppo, the entry of Turkey into the ground war, Turkish hostility toward U.S.-backed Kurdish rebel groups and the presence of American military advisers in widening conflict zones.

Meanwhile, in a blow to the opposition, rebel forces and civilians in the besieged Damascus suburb of Daraya were to be evacuated on Friday after agreeing to surrender the town late Thursday after four years of grueling bombardment and a crippling siege that left the sprawling area in ruins.

The surrender of Daraya, which became an early symbol of the nascent uprising against Assad, marks a success for his government, removing a persistent threat only a few miles from his seat of power.

Referring to Daraya, Lavrov said: "This is an example I think will get some following." He said the Russian military's reconciliation center in Syria has received a request from another area to organize a similar operation — with Russian mediation.

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Associated Press writer Vladimir Isachenkov contributed from Moscow.